Scaleable phase-locked segmented laser beam frequency shifter

ABSTRACT

This invention relates to a system for shifting the frequency and wavelength of a laser beam at high efficiency, both pulsed and continuous wave, said system consisting of a hexagonally packed array of optical fibers with non-linear optical cores (1) surrounding by cladding (2) having their end faces (3) optically polished. Input laser beam (4) is split via micro lens array (5) into an array of micro laser beams (6) which matches array of cores (1) where they are frequency converted, said converted beams emerging as micro beam array (7) which is collimated by the second micro lens array (8) into the phase-locked output beam (9).

FIELD OF THE INVENTION

This invention relates to a system for shifting the frequency andwavelength of a laser beam consisting of a scaleable, phase-locked arrayof non-linear optical fibers each of the said fibers being embedded inan optically transparent medium, which, collectively form a coherentlypacked bundle, with the opposite faces of said bundle formed by the endsof said fibers, being optically polished, with a micro lens arraypositioned near to each of the said optically polished faces such that alaser beam incident on one of the micro lens array is split into anarray of focussed micro laser beams each one of which matches into aparticular optical fiber in the array of said fibers forming the saidinvention. On being focussed into said fibers within said fiber bundleof said system, the laser light propagating through said fibers ispartially frequency shifted to a frequency which depends on theproperties of said non-linear optical medium forming said fiber coresand on the frequency of the input laser beam propagating through thesaid system.

The invention has application for the frequency and wavelength shiftingof laser beams used in materials processing, medical, research, defenceapplications and isotope separation.

SUMMARY OF THE PRIOR ART

Prior art laser beam frequency shifting systems consisted either of asingle crystal block of non-linear optical media or a single opticalfiber whose core consisted of a non-linear optical medium.

These prior art frequency shifting systems were ineffective in providingscaleable output power under continuous wave operation which demandsthat the laser beam intensities within the non-linear optical medium bevery high, exceeding about 100,000 watts per square centimeter, powerlevels which would melt known bulk media at such high beam intensities.

Prior art frequency shifting systems which utilised single fiber coresdid achieve high beam intensities over extended lengths, a process whichled to the efficient conversion (over 50%) of the primary laser beampropagating through said non-linear fiber, but at very low power levelsof only a few milliwatts.

The present invention overcomes the defects of prior art frequency andwavelength shifting systems in that it provides high beam intensitieswithin a phase-locked array of fibers of non-linear media so thatscalable laser beams of high, continuous wave power is achieved for thefirst time.

The present invention also differs from the frequency shifters of theprior art because it utilises a phase-locked array of laser beams whoserelative phases can be varied to produce scannable output beams at theshifted frequency.

BACKGROUND OF THE INVENTION

As scaleable, high power, continuous wave lasers have become available,their available wavelengths have tended to be in the near and midinfra-red regions of the electro-magnetic spectrum rather than in thevisible and ultra violet regions where an increasing number ofapplications are emerging.

In order effectively shift the infra-red laser wavelengths into thevisible and ultra violet regions under continuous wave conditions, it isnecessary to pass the said infra-red laser beams at high intensitiesover relatively long lengths of non-linear optical media. Thisrequirement eliminates the use of bulky non-linear crystals, which areeffective under pulsed conditions, because the enormous laser beamintensities would heat up the crystal, initially causing thermallyinduced self-focussing effects and finally simply destroying the saidbulk crystal.

It was known since the early 1960s that laser beams traversing the coresof multimode optical fibers gave rise to very high flux densities ofmany hundreds of watts per square centimeter. With the subsequent adventof single mode fibers, it was possible to inject a laser beam into acore only a few microns in diameter, that is, with a cross-sectionalarea of less than a millionth of a square centimeter so that less than awatt of continuous wave power was needed to achieve the enormous laserbeam intensities exceeding one megawatt per centimeter square.Furthermore, with what is essentially a laser beam of merely onewavelength diameter traversing the core of the single mode fiber, it wasnot possible for self-focussing effects to occur and the heat dissipatedper unit length of the fiber core was found to be negligible compared tothe heating effects in bulk materials of similar thermal conductivity.It was eventually observed that, at the high laser beam intensitieswhich could be maintained in single mode optical fiber cores, (normallya glass medium with linear optical properties), the fiber cores couldbecome a non-linear frequency shifting medium for the transmitted laserbeam. On the other hand, with fiber cores formed from non-linear opticalmaterial it became possible to convert over fifty percent of thepropagating primary laser beam into its harmonics in the said fibercores. However, once out of the fiber core, the laser beam expanded atangles of thirty degrees or more so that their intensities became verysmall as well as their total power which in general would be less thanone tenth of a watt for continuous waves.

The present invention provides for the generation of scaleable power,frequency shifted laser beams which are phase-locked together to formwhat is essentially a single laser beam. In other words, if one insertsthe invention into a given laser beam, then its frequency shiftedcomponent will be produced along the propagation path of the primarybeam or at the appropriate angle to said primary beam if the relativephases of the primary beam are adjusted.

The invention can take the form of a bundle of single mode opticalfibers with the appropriate non-linear cores or a solid block drilledwith an array of holes into which non-linear fiber cores are inserted.Fluid cooling of the matrix medium can be used to maintain equal pathlengths over the several centimeters of non-linear fiber cores requiredto achieve efficient conversion of continuous wave laser beam frequency.The invention can be positioned between two reflecting mirrorsappropriately mirrored to generate standing waves within the saidnon-linear fiber cores. The invention can also be operated under pulsedas well as continuous wave conditions.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a scaleable system forshifting the frequency and wavelengths of scalable power laser beams soas to achieve high conversion efficiencies under both pulsed andcontinuous wave operation without damaging the non-linear optical mediumin which the said frequency and wavelength shifting takes place.

Another object of the system is to provide for the phase-locking of thelaser beams so that the frequency shifted output beam consists of alarge number of phase-locked micro laser beams which are effectivelycombined into a single beam comparable in dimensions to that of theprimary input beam.

It is an object of the invention to split the primary input laser beamwhose frequency is to be shifted into an array of focussed micro beams,each of which matches the cores of an array of non-linear optical fibersso that each of the said micro beams propagates along its matching fibercores at intensities and over lengths of said fiber so as to provide forefficient frequency shifting, said frequency shifted laser micro beamemerging through the optically polished end face of the said fiber coreso that it is phase-locked with its neighbouring micro beams which arethen collimated via a second micro lens array to emerge as a single,phase-locked, frequency shifted laser beam.

Another object of the invention is to provide a reflecting mirror atboth ends to generate standing waves of both the primary and frequencyshifted laser beams within the cores of said single mode fibers.

It is also an object of the invention to transmit and frequency shiftlaser beams whose phases are adjusted such that the output frequencyshifted beam is scanned relative to the axis of the input beam.

Another object of the invention is to provide cooling channels so thatsaid system can be maintained at an optical path length consistent withgood phase-locking at elevated power levels.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the invention may be obtained from thefollowing considerations taken in conjunction with the accompanyingdrawings which are not meant to limit the scope of the invention in anyway.

FIG. 1 shows a cross-section of the invention with the input laser beambeing split into an array of focussed micro laser beams with a microlens array so that they are matched into an array of non-linear opticalfibers the frequency shifted output of which is collimated by a secondarray of micro lenses into a single, phase-locked, frequency shiftedlaser beam.

FIG. 2 shows the array of non-linear optical fibers contained within ablock of optically transparent material at the laser beam wavelength.

FIG. 3 shows the invention inserted between two laser mirrors to formand optical parametric oscillator.

FIG. 4 shows the packing of the non-linear fibers in a hexagonal formatresulting in the cores of the said fibers being equidistant from eachother with their core diameter being much less than the thickness oftheir cladding.

FIG. 5 shows the non-linear optical fibers in a hexagonally packed arraywith their core diameters greater than their separation.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1, numeral 1 indicates the non-linear optical medium in the formof a single mode optical fiber core whose cladding is indicated bynumeral 2. Numeral 3 indicates the optically polished andanti-reflection coated end faces of the non-linear optical fiber formedby non-linear core 1 and its cladding 2. Numeral 4 indicates the inputlaser beam which is split into an array of focussed micro beams usingthe micro lens array indicated by numeral 5. The focussed micro laserbeams indicated by numeral 6 enter non-linear medium 1 so that theirintensity is increased to several megawatts per square centimeter atwhich level a portion of the said laser beam is frequency shifted in amanner characteristic of non-linear medium 1 and the fundamentalfrequency of the incoming laser beam. The frequency shifted (and nonfrequency shifted) portion of the said laser beam emerge from the exitface of the said fiber array as the diverging micro laser beam arrayindicated by numeral 7 to be collimated by the micro lens arrayindicated by numeral 8 into the phase-locked output beam indicated bynumeral 9.

In FIG. 2, numeral 10 indicates the cross-section of a block containingan array of the non-linear optical media 11.

In FIG. 3, numeral 12 indicates a laser mirror which is fullytransparent to the input laser beam indicated by numeral 4 but fullyreflecting the frequency shifted laser beam generated by the invention.Numeral 7 indicates an array of diverging micro laser beams which arecollimated via micro lens array indicated by numeral 8 into a parallel,phase-locked laser beam which is partially reflected by the mirrorindicated by numeral 13 into a phase-locked output beam at the shiftedwavelength indicated by numeral 14.

In FIG. 4, numeral 15 indicates the non-linear medium with a corediameter smaller than the cladding thickness indicated by numeral 16.

In FIG. 5, numeral 17 indicates the non-linear medium with a corediameter larger than the cladding thickness indicated by numeral 18.

I claim,
 1. A scaleable system for shifting the frequency and wavelengthof a pulsed and continuous laser beam at high efficiencies, said systemconsisting of:(a) a bundled, hexagonally packed array of single mode,optical fibers with non-linear optical cores, said fiber bundle havingboth of its end faces optically polished and anti-reflection coated tominimise optical losses, the spacing between the said fibers within saidbundles being used as cooling channels, (b) a micro lens array to splitup the input beam into an array of focussed micro laser beams such thateach of the said micro laser beams matches a particular fiber core inthe said fiber bundle array, (c) a micro lens array which combines thearray of frequency shifted micro laser beams emerging from the exit faceof said fiber bundle so that they are collimated into a singlephase-locked laser beam at the shifted frequency.
 2. A system as claimedin claim 1, inserted between two mirrors, the first of the said mirrorsbeing fully reflecting at the frequency shifted wavelength and fullytransmitting at the input laser beam frequency whilst the second mirror,positioned near the output end of said system, is fully reflecting atthe input laser beam frequency and partially reflecting at the shiftedfrequency of the output beam.
 3. A system as claimed in claim 1 whichcan be scaled by adding more fibers to said fiber bundle and enlargingthe matching micro lens arrays.
 4. A system as claimed in claim 2 whichcan be scaled by adding more fibers to said fiber bundle and enlargingthe matching micro lens arrays and mirrors.
 5. A scaleable system forshifting the frequency and wavelength of a pulsed and continuous laserbeam at high efficiencies, said system consisting of:(a) a solid blockof material with its opposite end faces being optically polished, saidblock being drilled to form a parallel array of holes between said endfaces into which are inserted fibers, with optically polished ends, ofnon-linear optical material to form a close packed array of said fibers,(b) a micro lens array to split up the input beam into an array offocussed micro laser beams such that each of the said micro laser beamsmatches a particular fiber core in the said block, (c) a micro lensarray which combines the array of frequency shifted micro laser beamsemerging from the exit face of said block so that they can be collimatedinto a single, phase-locked laser beam at their shifted frequency.
 6. Asystem as claimed in claim 5, inserted between two mirrors, the first ofthe said mirrors being fully reflecting at the frequency shiftedwavelength and fully transmitting at the input laser beam frequencywhilst the second mirror, positioned near the output end of the saidsystem, is fully reflecting at the input laser beam frequency andpartially reflecting at the shifted frequency of the output beam.
 7. Asystem as claimed in claim 5 which can be scaled by drilling more holesin said block and enlarging its size allowing for the insertion of moreof the said fibers in the process, whilst allowing from the enlargementof the matching micro lens arrays in the process.
 8. A system as claimedin claims 1 and 5 in which the micro laser beams propogating throughsaid fiber cores have their phase delayed with respect to each other sothat their output beams are scanned relative to the direction ofpropogation of the input beam.